OPC UA is a communications protocol coming from the OT industry. It is both open-source and cross-platform available. It is a protocol put in place to standardize communications between IoT devices in industrial setting. One of its biggest strengths is that the machine data can be semantically described in a machine-readable way, which allows for uncommon communication paths. It also allows for more sophisticated object modelling than the MQTT protocols. Problem of this protocol is that because of its abundance of features it is difficult to get a grasp onto it, especially compared to the simplistic design of MQTT. Also, scalability is an issue due to the bulky nature of the protocol.
If you would like to know more, you could watch the following video or read the several hundred pages of OPC UA specifications to get a grasp on the scope of it:
